Today's machines are relied upon to operate with minimal attention. Many industrial and commercial facilities operate hundreds or even thousands of machines concurrently, many of which are integrated in a large interdependent process or system. Although maintenance procedures are becoming increasingly efficient, at any time at least a small percentage of the machines are prone to failure.
For example, machines having moving parts, such as bearings, are subject to constant friction that result in wear. For example, numerous studies have shown that bearing failures are the number one cause of motor faults. Unfortunately, however, most wear sites are concealed in the machine's assembled state. In particular, bearing damage due to wear from inadequate lubrication, shock or lubricant contamination may not be immediately apparent absent gross damage and/or failure. Thus, it is difficult to monitor wear rates and to prevent excessive wear on internal components of a machine.
Vibration analysis is an established nonintrusive technique for measuring the health of mechanical components in rotating machines. Every rotating machine exhibits a characteristic vibration signature which varies with the design, manufacture, application and wear of each component. Vibration may be generated by machine bearings including, for example, the bearing mounting, balls and ball races, misalignment of gears, motors, or shafts, and imbalance of rotors, gears, impellars and fans. Analysis of a machine's vibration signature is valuable for reducing unscheduled down time, reducing downtime for repair, minimizing periodic disassembly of a machine for inspection and greatly reducing the probability of catastrophic and unexpected machine failure.
A machine's vibration signature is composed of the sum of the vibration signals produced by and/or transmitted through each component of the machine. The vibration signals produced by a component includes forcing frequencies that vary with the rotational speed of the machine. For example, the forcing frequencies for a bearing include those of the inner race, the outer race and the ball track, and can be calculated as a function of the rotational velocity, the ball diameter, the pitch diameter, the contact angle and the number of balls. The forcing frequencies are sometimes referred to as the critical frequencies. The health of a particular component can be analyzed by considering the shape and magnitude of the vibration signals at the critical frequency or at harmonics of the critical frequency.
In analyzing the signature of a particular machine, the vibration signal of a particular component can be compared to signals from identical machines and/or historical signals to determine differences between signals or changes over time that may indicate a problem. Unfortunately, although the critical frequencies are readily calculable, measurement errors and the combination of the vibration signals from the different components and other sources mean that the signal-to-noise ratio at the frequency of interest may not be sufficient to perform an accurate analysis.
Furthermore, between a vibration sensor and the source of vibration, such as the bearing mentioned above, there is a mechanical transmission path which transmits the vibration signals from the source, such as a bearing, to the sensor. Each machine also has natural or resonant frequencies which cause relatively large increases in the amplitude of vibration for a given force input. If the resonant frequency of the transmission path is known, the signals will be amplified by that resonant frequency.
Thus the problem identified above of poor signal-to-noise ratios at a critical frequency can be overcome by looking at a harmonic of the critical frequency near the resonant frequency of the relevant transmission path. Resonant frequencies are generally a function of mass, stiffness and damping of a structure, however, and every machine and every transmission path will have a different set of resonant frequencies. As a result, the particular resonant frequency of a given transmission path is difficult to calculate without empirical measurements.
A calibrated force hammer and a vibration sensor (such as an accelerometer) generally are used to find the resonant frequency. A calibrated force hammer with a force transducer is used to strike the machine and to excite the structure with an approximated impulse signal to determine the resonant frequencies.
Generally, the test is performed as follows. With the machine off, a relatively small hammer is used to strike a few blows to the machine. A force transducer in the hammer sends a signal containing impact data to a spectrum analyzer, and the vibration sensor mounted on the machine sends frequency response data to the spectrum analyzer. By hitting the structure of the machine near points of interest (such as near the bearing cage) the resonant frequency of the transmission path of the structure between the hammer blow and the vibration sensor may be approximated.
The transient response generated by the hammer blow is measured with the spectrum analyzer to calculate the resonant frequency for that transmission path. In a time domain plot, the response appears as a large spike in amplitude which dies down over time. In a frequency domain plot, the graph is relatively flat with a noticeably high amplitude at a particular frequency, the resonant frequency. One advantage of the impact test is that a range of frequencies are excited relatively equally, and the resonant frequency stands out clearly.
Although sufficiently accurate for some applications, this procedure is imprecise and often impractical. For example, although measurement of resonant frequencies may be performed in the factory before the machine is delivered to a customer, many circumstances may lead to a machine in the field with an unknown resonant frequency where striking the motor with a calibrated hammer is inconvenient or very difficult. Furthermore, the procedure includes reproducing several hammer blows in the same spot to produce an average response over time (to reduce noise), the hammer must necessarily strike an outer housing of the machine rather than at the location of the vibration source (such as a bearing) inside the housing, and the excitation frequencies generated by the hammer blow are dependent on the hardness of the hammer face. Thus if the proper hammer is not used, the resonant frequency is not excited and the test must be repeated with a hammer having a different face hardness.